Lecture 7: Vestibular system Flashcards
What is the function of the vestibular system
Provides sense of balance with respect to placement of head in space. and designed to sense motions that arise from head movements as well as the inertial effects due to gravity.
It is generally unconscious, but essential for co-ordination of motor responses, eye movements and posture.
What causes nausea when you get sea sick?
Unnatural movements of head, produce a conflict
between visual and vestibular input which leads to disorientation and nausea (reticular formation).
Describe the two components of the vestibular labyrinth and the liquids inside.
Bony labyrinth: Convoluted hard walled cave of canals within petrous portion of temporal bone. Filled with perilymph (low K+ / high Na+, similar to CSF).
Perilymph secreted by arterioles in periosteum surrounding labyrinth.
Drains into subarachnoid space via perilymphatic duct which runs through cochlear aqueduct in temporal bone.
Membranous labyrinth: Convoluted delicate walled sac of ‘ducts’ floating in perilymph and following shape of bony labyrinth canals. Filled with endolymph (high K+ / low Na+, similar to intracellular fluid).
Endolymph is secreted by tissue in cochlear duct.
Drains into an extradural sac via endolymphatic duct which runs through vestibular aqueduct in temporal bone.
What is meniere’s disease?
Pressure balances in perilymph and endolymph important for functioning of vestibular system. Excess pressure causes vestibular disturbance: Meniere’s disease
What are the two otolith organs? What is their primary function? What movements do they detect?
Utricle and Saccule - their primary function is to detect movements dependent on gravity. Respond to changes in angle (tilt) and linear movements of the head and are maximally sensitive to straight line changes in acceleration and direction.
The utricle detects tilt and linear movements in the transverse plane. Moving forward and back, and left and right.
For example: doing a handstand, riding a bike, swaying side to side.
Name the components of the membranous labyrinth
In each inner ear there are:
3 semicircular ‘ducts’ in semicircular ‘canals’:
Anterior semicircular canal
Lateral semicircular canal
Posterior semicircular canal
2 otolith organs in vestibule:
Utricle
Saccule
What movements are the semicircular canals maximally sensitive to?
Sensory receptors in semicircular canals are maximally sensitive to rotational motions that lie in the plane of the canal.
Name the semicircular canals and the movements they detect (with example activities)
Anterior:
Detects rotations of head in sagittal plane.
Nodding head forward and back as in ‘yes’.
Doing a somersault.
Lateral:
Detects rotations of head in transverse plane.
Turning head side to side as in ‘no’.
Doing a pirouette.
Posterior:
Detects rotations of head in coronal plane.
Tilt head towards shoulder as in ‘maybe’.
Doing a cartwheel.
Where are sensory receptors located in the semicircular canal?
Sensory receptors in semicircular canals are located in the ampullae.
(which are dilated ends of each canal near their attachment to utricle inside vestibule.)
At the ampulla endolymph is partitioned off by flexible septum of receptive tissue called the crista ampullaris.
Describe the structure of the crista ampullaris
Ridge of crista ampullaris projects into lumen of ampulla of semicircular canal and is bathed in endolymph.
Covered by neuroepithelium made of sensory hair cells and supporting cells.
Sensory hair cells’ cilia embedded in gelatinous mass called the cupula.
Hair cells are either excited or inhibited when cupula sways in endolymph and moves with respect to neuroepithelium (see hair cell functional anatomy).
Upon rotational acceleration of the head, the endolymph in the semicircular canals which are oriented in the plane of the acceleration, is subjected to inertia and bends cupula.
Describe the biophysics of the semicircular canals
At the beginning of head rotation within the plane of a semicircular canal (ie. spinning = rotational acceleration in transverse plane = lateral canal) endolymph is subjected to inertia and lags behind in canal and so endolymph temporarily moves in opposite direction to head rotation.
Cupula has same specific gravity as endolymph and so ‘sways’ in same direction as endolymph as it experiences inertia. Swaying cupula effectively bends the sensory hair cells embedded in it in the opposite direction to the direction of head rotation.
Think marshmallow floating in a cup of hot chocolate.
When gently rotate cup, marshmallow doesn’t initially
follow the cup because it’s floating on hot chocolate
and both are subject to inertia … so they lag behind
the cup rotation. … Imagine what would happen if
The marshmallow was melted to the side of the cup
Describe the movements that the otolith organs detect and the location of their receptors.
Utricle: Detects tilt / linear movements of head in transverse plane.
Moving forward and back. Moving left and right.
Doing a handstand. Riding a bike. Swaying side to side.
Receptive tissue ‘patch’ positioned horizontally on floor.
Receptors oriented to detect linear movements in transverse plane.
Saccule: Detects linear movements of head in sagittal plane.
Moving up and down.
Jumping. Riding an elevator.
Receptive tissue ‘patch’ positioned vertically on medial wall.
Receptors oriented to detect linear movements in sagittal plane.
What is the name of the receptive tissue patches in otolith organs?
They are called maculae. One it located on the floor of the utricle (macula utriculi) which wobbles in the transverse plane. One is located on the wall of the saccule (macula sacculi) which wobbles in the saggittal lplane .
What makes otolith organs more sensitive to gravity rather than rotational motions like the semicircular canals?
Unlike the semicircular canals, where hair cells embedded in the gelatinous mass - the cupula. Movement of endolymph due to rotation of the head causes this to move.
The maculae of the otolith organs are heavier than endolymph so are more responsive to changes in gravity rather than movement of endolymph.
Describe the structure of the macula
Patches of maculae project off floor / wall into cavities of utricle / saccule and are bathed in endolymph (but not moved by endolymph).
Composed of neuroepithelium made of sensory hair cells and supporting cells.
Sensory hair cells’ cilia embedded in gelatinous mass called the otolithic membrane and this is studded with ‘weighty’ calcium carbonate otoliths.
Hair cells are either excited or inhibited when otolithic membrane moves with respect to neuroepithelium (see hair cell functional anatomy). Because otolithic membrane is weighted, it is subject to gravitational changes (pulled down when head tilts) as well as to inertia (lags behind when head accelerates).
